Product containing high density skins

ABSTRACT

A fiberboard is manufactured having relatively high density skins on a relatively low density core by including urea in at least the surface fibers of a consolidated mat, having a density of less than 35 pounds per cubic foot, and then hot-pressing the consolidated mat at a temperature of at least 525° F. to form a board having high density surface skins. In accordance with another embodiment of the present invention, urea is used to form a skin on higher density mats having a density of 35 pounds/ft. 3  or greater. The skin formed on the higher density mats has strength and density properties which are not as significantly different from the core material as is the skin formed over the low density core material of the first embodiment, but the skin on the higher density mats also improves the board strength, stiffness, paint holdout and design fidelity properties.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of our prior application Ser. No. 739,184, filed Nov. 5, 1976; now U.S. Pat. No. 4,175,150.

FIELD OF THE INVENTION

In accordance with one embodiment of the present invention, the invention relates to a fiberboard panel having a relatively low density core or central portion and having relatively high density skins or surface portions. In accordance with another embodiment of the present invention, a consolidated mat having a density of 35 pounds/ft.³ or greater is provided with a high density skin to improve the surface properties of strength, stiffness, paint holdout and design fidelity. The skin formed has a density of about 40-55 pounds/ft.³ and, therefore, may or may not be more dense than the core material. More particularly, the present invention relates to a man-made board having high density skins or surface portions as an integral surface part of a fiberboard or particle board panel or panel portion wherein the high density skins are produced by contacting the surface fibers of a consolidated board with urea, and thereafter hot-pressing the board in a post-press or second pressing operation at a temperature of at least 525° F. to form smooth, stiff, hard, dense skins on the surface fibers contacted with urea.

BACKGROUND OF THE INVENTION

The process of the present invention relates to a "post-press" or after consolidation press operation for creating thick, hard, integral surface portions on at least one and preferably both surfaces of a man-made fiberboard or particle board substrate. A handleable mat is first manufactured by consolidating a cellulosic fiber-containing composition to form a consolidated fiberboard or particle board. The handleable mat can be consolidated by drying a mat formed by depositing a water slurry of fibers onto a water pervious support member, as is common in the manufacture of insulation board, or the mat can be consolidated under heat and pressure until substantially dry in a first press operation. This consolidated mat is thereafter "post-pressed" to form a surface skin to improve the stiffness, strength, paint holdout and design fidelity properties of the board. In accordance with one embodiment disclosed in our prior U.S. Pat. No. 4,175,150 a low density core material of less than 35 pounds/ft.³ is maintained and a skin, formed in the after consolidation press step with urea, is a higher density surface portion. In accordance with a second embodiment disclosed herein, heavier skinned surface products are formed in the same manner but starting with higher density consolidated mats on the order of 35 pounds/ft.³ or greater, for example, 35-65 pounds/ft.³. For the purpose of the present invention, a "post-press" operation refers to a press step performed on the mat after consolidation thereof.

The fiberboard of the present invention is generally formed in two separate operations. The first operation forms a consolidated mat while the second or "post-press" operation creates the denser surface portions. It should be noted however, that consolidated man-made board previously manufactured are readily available and can be used as the consolidated mat to impart a skin to such consolidated mats in accordance with the present invention. It is quite surprising that a "post-press" or second pressing operation on a consolidated mat surface treated with urea is effective in forming a smooth surface skin on the mat. Prior to the present invention, it was thought that the qualities and physical characteristics of a board would be finally determined by the conditions under which the board is initially consolidated and by the raw materials used to form the board, because of the bonding occuring during consolidation.

Much of the bonding necessary for cohesion and strength in a man-made board occurs during the consolidation of the board during hot-pressing. Prior to drying a board made by the wet process, the board is weak, but the bonding forces created during the final consolidation are generally quite powerful. Generally the same bonding effect occurs in the final consolidation of a board made by the dry process. Prior to hot-pressing, the loosely disassociated cellulosic fibrous product is quite weak, but after hot-pressing the mat into its final configuration, it is very powerfully held together by the bonding which occurs during hot-pressing. Hot-pressing during consolidation causes a welding or coalescing of the cellulose fibers at the surface of the product so that the surface portion consists of wood remade in modified form. It is very difficult to densify or restructure the surface of a consolidated product without again destroying these fiber-to-fiber surface welds. It is especially difficult to densify only a surface portion of a consolidated board in an amount sufficient to substantially increase the strength and stiffness of the product while retaining the lightweight core material to provide a lightweight product. This same concept can be used to improve and restructure the surface of higher density core materials by post-pressing consolidated mats having a density of 35 pounds/ft.³ or greater. The process described herein accomplishes these results.

A prior patent, assigned to Masonite, U.S. Pat. No. 4,007,076, discloses and claims a method of post-press embossing consolidated, man-made boards to effectively and accurately reshape the surface of a completed panel without surface defects such as surface fiber tearing. In accordance with U.S. Pat. No. 4,007,076, a previously consolidated panel is post-press embossed at a pressure of at least 1000 psi to create a surface design, or to permanently redesign the surface of a consolidated product without destroying the interfiber bonding established at the surface of the board during consolidation. The product thus formed does not include a skin formed by post-pressing with urea and does not disclose retaining a relatively low density core or central portion in accordance with one embodiment of the present invention.

In accordance with the present invention, it has been found that the surface physical characteristics of a consolidated mat can be altered in a post-press operation to harden, smooth, and densify the surfaces thereof when at least the surface fibers of the mat are contacted with urea. During the post-press operation, at a temperature of at least 525° F., the urea reacts to stiffen and strengthen the cellulosic substrate, such as insulation board and fiberboard, to provide the necessary strength for the intended purpose of the substrate while, in one embodiment, retaining relatively low density, lightweight central core of the material and in another embodiment, using a consolidated mat of at least 35 pounds/ft.³ density, improving the stiffness, strength and surface properties of paint holdout and design fidelity.

PRIOR ART

An extensive search was performed to determine the prior art use of urea in the manufacture of cellulose fiber-containing substrates. Various patents and abstracts were found relating to the use of urea for fire-retardance, plasticization, resistance to aging, and as a binder. None of these patents or abstracts found, however, relates to the use of urea as disclosed herein, in post-pressing of a man-made cellulose fiber-containing product to provide a relatively high density, hard, stiff skin on one or more surfaces of a fiberboard or particle board. The relevant patents and abstracts found in the search are as follows:

    ______________________________________                                         A.B.I.P.C. Abstracts Patents                                                   ______________________________________                                         Vol. 36, No. 4; 2483; 1965                                                                          2,912,392                                                 Vol. 38, No. 6; 4917; 1967                                                                          2,912,394                                                 Vol. 39, No. 9; 7630; 1969                                                                          3,285,801                                                 Vol. 39, No. 11; 9657; 1969                                                                         3,667,999                                                 Vol. 41, No. 5; 4311; 1970                                                                          3,676,389                                                 Vol. 42, July-Dec.; 5715; 1971                                                                      3,779,861                                                 Vol. 43, No. 9; 9665; 1973                                                                          3,790,442                                                 Vol. 44, No. 10; 10423; 1974                                                                        3,881,992                                                 Vol. 44, No. 10; 10754; 1974                                                                        3,915,911                                                 Vol. 45, July-Dec.; 4724; 1974                                                                        777,090 Canada                                          Vol. 45, No. 2; 1479; 1974                                                     Vol. 46, No. 3; 2233, 1975                                                     Vol. 46, N9. 5; 4954; 1975                                                     ______________________________________                                    

SUMMARY OF THE INVENTION

An object of the present invention is to provide a lightweight fiberboard or particle board panel having hard, dense surface skins thereon.

Another object of the present invention is to provide a cellulosic fiber-containing article having a hard, dense smooth skin on at least one surface thereof.

Another object of the present invention is to provide a lightweight fiberboard, having a central core of material with a density in the range of 10-35 pounds/ft.³, having strength and stiffness properties equivalent or better than hardboard having a density in the range of 45-65 pounds/ft.³

Another object of the present invention is to create a skinned surface on a consolidated mat having a density of at least 35 pounds/ft.³, for example 35-65 pound/ft.³, to create a relatively dense fiberboard or particle board having excellent strength, stiffness, paint holdout and design fidelity properties.

Another object of the present invention is to provide a lightweight man-made board having design-fidelity and paint hold-out properties equal to or better than hardboards manufactured in accordance with existing technology.

Another object of the present invention is to provide a method of providing an integral, structural skin on one or more surfaces of a cellulosic fiber-containing consolidated product by including urea within at least the surface fibers of the consolidated product, and, thereafter, post-pressing the urea treated consolidated board in an after consolidation or post-pressing operation at a temperature of at least 525° F.

In accordance with an important feature of the present invention, it has been found that urea will provide hard, dense surface skins to a consolidated handleable mat when the surface fibers are contacted therewith and the mat is thereafter "post-pressed" at a temperature of at least 525° F.

In accordance with another important feature of the present invention, the technology disclosed herein has been developed to provide a low density, strong cellulosic fiber-containing product, for example, fiberboard which has sufficient strength for end uses such as wall panelling, siding, and moldings. The low density, high strength product is produced by first manufacturing or otherwise obtaining a low density consolidated fiberboard or particle board substrate having strength sufficient to be handled in manufacture, including urea in at least the surface fibers of the substrate and thereafter post-pressing the substrate to develop dense outer layers or skins on the exterior surfaces.

In accordance with another important feature of the present invention, a higher density consolidated mat having a density of at least 35 pounds/ft.³ is provided with a skin, defined as a composition formed by hot-pressing a consolidated cellulosic fiberboard or cellulosic particle board containing urea in at least the surface being hot-pressed at a temperature of at least 525° F. Such hot pressing of consolidated mats containing urea creates a smooth skin having a density in the range of about 40 to about 55 pounds/ft.³ and is formed in a thickness of about 1 to about 7 mils.

Surprisingly, it has been found that the skin created by post-pressing a low density consolidated product having urea in at least the outer surface fibers thereof creates a hard, dense surface which, if desired, effectively and permanently reproduces embossing plate details on the surface of the product and creates a smooth outer surface having excellent holdout of coating materials, such as paint. The post-pressing operation restructures the surface fibers contacted with urea to provide exceptional strength and stiffness properties to a lightweight panel. The panel manufactured in accordance with the present invention from a relatively lightweight consolidated mat having a density of less than 35 pounds/ft.³ compares in material costs and physical properties, such as breaking load and stiffness, to one-quarter inch hardboard panelling having a density of about 60 pounds/ft.³. One-quarter inch hardboard was used as the control to determine the viability of producing the low density product of the present invention having high density, high strength skins as surface layers.

DETAILED DESCRIPTION OF THE INVENTION Consolidated Mat

In accordance with one embodiment of the present invention, a high strength fiberboard product is manufactured having hard, dense skins or surface portions thereon over a relatively low density core material. A low density consolidated mat having a density less than about 35 pounds/ft.³, i.e. 10-35 pounds/ft.³, is manufactured using either the wet or dry process as known in the manufacture of man-made boards. The low density consolidated mat is then post-pressed at a temperature of at least 525° F. after first treating at least the surface fibers of the mat with urea to form a skin on the surfaces of the product.

The method of producing a consolidated mat is well known as presently used in producing man-made boards such as hardboard, chipboard, particle board, panelboard, acoustical board, insulation board, and the like. In the wet process, the raw material is uniformly blended in a head box with copious quantities of water to form a slurry. The slurry is deposited onto a water-pervious support member, generally a Fourdrinier wire, where much of the water is removed leaving a wet mat of cellulosic material. The wet mat is then dryed to consolidate, as in the manufacture of insulation board, or can be transferred from the pervious support member and consolidated under heat and pressure to form the board. Typically, pressures from 400 to 500 psi and temperatures up to about 400° F. are encountered in hot-press consolidation of a man-made board manufactured by the wet process. The dry process is similar to the wet process except that the cellulosic fibers are first coated with a thermosetting resin binder, such as a phenol-formaldehyde resin, and are then randomly distributed into a mat by air blowing the resin-coated fibers onto a support member. In the dry process, the mat is pressed at temperatures up to about 450° F. and pressures less than about 1000 psi to cure the thermosetting resin and to compress the mat into an integral consolidated structure.

The lightweight handleable mat on which a skin is formed in accordance with one embodiment of the present invention is manufactured in a conventional manner, using conventional cellulosic fiber stock. To achieve a lightweight product having strong, stiff, more dense skin surfaces, the handleable mat should have a density, after consolidation, in the range of about 10-35 pounds per cubic foot, preferably in the range of 15-30 pounds/cubic foot. Panels of varying thicknesses having densities within this range can be produced in accordance with known technology to provide lightweight core materials on which a surface skin can be developed in accordance with one embodiment of the present invention.

Handleable mats have been produced having a density as low as ten pounds per cubic foot and a thickness of one and one-half inches. Mats having densities as low as ten pounds per cubic foot are useful as the handleable mat in producing lightweight products in accordance with the principles of the present invention. Any low density mat (less than 35 pounds per cubic foot) is considered to be handleable if it can survive the trimming, cutting, stacking, packing, shipping, and unloading operations necessary to produce fiberboard. All such handleable mats are useful in accordance with the principles of the present invention.

In accordance with a second embodiment of the present invention, heavier fiberboard or particle board consolidated mats having densities of at least 35 pounds/ft.³, i.e. 35-65 pounds/ft.³ can be provided with a surface skin, although the surface skin may or may not be more dense than the cellulosic material on which the skin is formed, particularly for fiberboard or particle board consolidated mats having a density of 40 pounds/ft.³ or more. The heavier fiberboard or particle board consolidated mats also are manufactured in a conventional manner using conventional cellulosic fiber stock. Fiberboard or particle board consolidated panels having a density of 35-65 pounds/ft.³ can be produced in accordance with known technology and urea included in at least the surface fibers either before or after consolidation. The consolidated mat containing urea then is post-pressed at a temperature of at least 525° F. to form a smooth surface skin of unknown composition.

Skin Development

The consolidated mat containing urea is pressed in a post-pressing operation at a temperature of at least 525° F. to develop a surface layer herein called a "skin", defined as an outer layer having a different composition than a thickness of cellulosic material over which it is formed. It was found in early investigations that heat and pressure alone would not form a thick skin on the surface of a low density panel. The skin thickness is not as readily apparent on the surface of higher density panels because its properties, while different than the cellulosic layer thereunder, are not significantly visibly different since the density of skin and underlayer may be approximately equal. However, the skin certainly forms as a result of the high temperature post-pressing with urea and the skin on higher density panels significantly improves the paint holdout and design fidelity properties of the board. Table I shows that post-pressing alone, without urea pretreatment, does not produce a skin.

The table also shows that the sample post-pressed with urea had a higher breaking load and a higher stiffness than the untreated fiberboard sample by 46% and a stiffness better than the unpressed fiberboard sample by 82%. These property improvements occured even though the post-pressed sample was thinner. Normally it would be expected that the thicker product would have the higher stiffness and breaking load values.

                  TABLE I                                                          ______________________________________                                         Effect of Heat Pressure and Post-Pressing                                      On Skin Development                                                                                                     Break-                                          Final            Skin    Stiff-                                                                               ing                                             Caliper Density  Thickness                                                                              ness  Load*                                 Substrate (In.)   lb/ft.sup.3                                                                             (In.)   lb/in.                                                                               lb.                                   ______________________________________                                         fiberboard                                                                     not post-pressed                                                                         0.563   17       0       103   24                                    post-pressed                                                                             0.406   22       0        68   20                                    fiberboard                                                                     (no urea)                                                                      fiberboard                                                                     (w/urea and                                                                    post-pressed                                                                   at 550° F.)                                                                       0.406   22       0.030   188   35                                    ______________________________________                                          *The stiffness and breaking load data as set forth throughout this             disclosure, were obtained by breaking 3" × 12" samples over a 10"        span. A load was applied to the center of the span and the center of the       sample was deflected at a constant rate. As the sample was deflected, the      force (load) exerted on the sample was recorded. As the sample is bent,        the force is continuously increased until the sample breaks. A                 load/deflection curve can be obtained for the tested samples, as set fort      in the FIGURE. During the initial phases of loading, the force applied is      a linear function of the deflection. The term "stiffness", as used             throughout this disclosure, is the slope (lb/in.) of this linear portion       of the load deflection curve. The term "breaking load", as used throughou      this disclosure, is the maximum load point obtained on the load/deflectio      curve before the sample breaks.                                          

Various chemicals were also evaluated on the surface of low density mats in an attempt to increase the skin thickness. Chemicals attempted were phenol-formaldehyde resin, urea-formaldehyde resin, gelatin, mixtures of gelatin with phenol-formaldehyde, n-methylolacrylamide, and urea. Urea proved to be unexpectedly superior for skin development on the surface of a cellulosic fiber-containing substrate, particularly for low density (10-35 pounds/ft.³) fiberboard panels, as shown in Table II:

                                      TABLE II                                     __________________________________________________________________________     EFFECT OF CHEMICAL SURFACE SPRAY                                               Skin Thickness (In.) at Equivalent Cost Level                                             $5/MFt.sup.2 $10/MFt.sup.2 $20/MFt.sup.2                                       Pounds/                                                                               Skin          Skin          Skin                             Chemical   MFT.sup.2                                                                             Thickness                                                                            Pounds/MFt.sup.2                                                                       Thickness                                                                            Punds/MFt.sup.2                                                                        Thickness                                                                            REMARKS                    __________________________________________________________________________     urea       55     .026  111     .032  222     .033  Tough, smooth, but                                                             poor bond between                                                              skin and mat               gelatin/                                                                       phenol-formaldehyde                                                                       7      .015  15      .022  29      .030  Glossy surface film        urea-formaldehyde                                                                         42     .013  83      .019  167     .024  Hard surface but                                                               blistered in spots                                                             on                                                                             all boards                 n-methylol-                                                                    acrylamide 7      .014  14      .017  27      .018  Tough, smooth                                                                  surface                                                                        with good bond to                                                              mat                        gelatin    6      --    12      --    25      --    When exposed to hot                                                            platens the surface                                                            gummed up, charred,                                                            and stuck to platen        phenol-formaldehyde                                                                       24     .014  48      .014  95      .024  Reddish brown flat                                                             surface, tight bond                                                            to mat                     __________________________________________________________________________

In attempting to achieve thicker surface skins on the handleable mat during post-pressing it was found that urea should deeply penetrate the surface fibers of the handleable mat prior to post-pressing. Surface spraying therefore led to the impregnation method of incorporating urea into the handleable mat.

IMPREGNATION PROCESS

It has been found that when urea is impregnated into the surface of a consolidated mat having a density in the range of 10-65 pounds/ft.³ after initial consolidation of that mat, a much thicker skin can be produced by vacuum impregnating the urea to provide a skin thickness in the range of 0.060 to 0.070 inch. Surprisingly, urea is the only chemical impregnant able to achieve a skin thickness of 0.060 inch, regardless of the depth of penetration of the impregnant.

In testing various degrees of skin forming chemical penetration by vacuum impregnating the mat after initial consolidation, it was found that increasing the depth of skin forming chemical penetration beyond that required to produce 0.060 inch skins, for example, at a level of 111 pounds/thousand square feet, does not improve the physical properties of the panel, nor does it increase the skin thickness beyond 0.070 inches.

In accordance with the results achieved as set forth in Table III, each material was applied to the surface of a 20 pounds/cubic foot handleable mat at an equivalent cost level and a vacuum box was utilized on the under surface of the mat to insure deep penetration of the impregnant. After impregnation, each mat was dried so that the moisture content in the hot-press (post-press) did not exceed 10% (wet basis). The post-press conditions were as follows:

    ______________________________________                                         Platen Temperature  550° F. (Both Sides)                                Closing Speed       Fast as Possible                                           Holding Time at Caliper                                                                            30 Seconds                                                 Caliper             5/8"                                                       Pressure            5000 psi                                                   Opening Speed       Fast as Possible                                           ______________________________________                                    

                  TABLE III                                                        ______________________________________                                         Effect of Impregnation                                                                                    Skin                                                          Caliper Density  Thickness                                                                              % Increase in                               Treatment (In.)   (#/ft.sup.3)                                                                            (In.)   Skin Thickness                              ______________________________________                                         phenol-                                                                        formaldehyde                                                                             .615    26.5     .047    136                                         gelatin/                                                                       phenol-                                                                        formaldehyde                                                                             .611    28.8     .033    50                                          n-methylol-                                                                    acrylamide                                                                               .612    24.7     .031    82                                          urea formal-                                                                   dehyde    .633    25.0     .036    89                                          urea*     --      --       .060    88                                          ______________________________________                                          *Note:                                                                         Skins separated from urea treated mats after postpressing.               

It should be noted that while the urea was the best penetrant for achieving a thick, hard, dense, tough skin on the surface of a handleable mat, each of these urea skins delaminated upon post-press treatment so that a secondary, and a difficult problem of adhereing the skins to the core material remained.

Investigations to find a solution to the skin delamination problem led to the theory that perhaps urea penetrated into the core of material and disintegrated or otherwise interfered with the urea-formaldehyde binder at the interface, causing skin delaminations.

Handleable mats having a density of 25 pounds/cubic foot were formed containing 8% phenol-formaldehyde resin as the binder. Two such handleable mats were impregnated on each surface with urea in amounts of 28 and 55 pounds/thousand square feet/side. The phenol-formaldehyde containing handleable mats were then post-pressed under the following conditions:

    ______________________________________                                         Press Conditions:                                                              ______________________________________                                         Platen Temperature                                                                               550° F.                                               Closing Time      Fast as Possible                                             Pressure (Maximum)                                                                               200 psi                                                      Holding Time at Caliper                                                                          30 Seconds                                                   Caliper           5/8"                                                         Opening Time      Slow (10-15 Seconds)                                         ______________________________________                                    

Quite surprisingly, the mats did not delaminate after post-pressing. In comparison with the same board containing 8% urea-formaldehyde resin binder it was observed that the skins produced on the urea-formaldehyde binder boards were always thicker than the skins obtained on the phenol-formaldehyde binder boards, as set forth in Table IV:

                  TABLE IV                                                         ______________________________________                                         EFFECT OF MAT BINDER ON SKIN THICKNESS                                                        Urea Impregnation                                                                            Skin Thickness                                    Consolidated Mat Binder                                                                       (lbs/MFt.sup.2)                                                                              (inch)                                            ______________________________________                                         urea-formaldehyde                                                                             28            .032"                                                            55            .050"                                             phenol-formaldehyde                                                                           28            .024"                                                            55            .027"                                             ______________________________________                                    

Accordingly, the consolidated mat can be manufactured with a combination of resin binders to take advantage of the thicker skins achieved when a urea-formaldehyde binder is used and to take advantage of the absence of delamination when a second binder, such as phenol-formaldehyde, is employed. Further, it was found that when urea is combined by impregnating the handleable mat, the hardest (most abrasive) skin was provided by the combination of urea-formaldehyde with phenol-formaldehyde binders. Handleable mats were produced using, as a binder, phenol-formaldehyde resin and a combination (1:1 weight ratio) of phenol-formaldehyde resin with urea-formaldehyde resin--each of these mats was impregnated with urea on its surface and post-pressed without delamination. The consolidation and post-press conditions were as follows:

    ______________________________________                                         Consolidated Mat Manufacture (Dry Process):                                    1.  Urea-formaldehyde/phenol-formaldehyde binder system:                       Pre-press         50 psi                                                       Steam Pressure    90 psig                                                      Caliper           .625"                                                        Time at Caliper   6 minutes                                                    2.  Phenol-formaldehyde binder system:                                         Pre-press         50 psi                                                       Steam Pressure    550 psig                                                     Caliper           .625"                                                        Holding Time      7 minutes or as long                                                           as it takes for the                                                            core to reach 325° F.                                 Post-Press Conditions:                                                         Platen Temperature                                                                               550° F.                                               Caliper           .550"                                                        Holding Time      30 seconds                                                   ______________________________________                                    

The characteristics of the boards are set forth in Table V:

                                      TABLE V                                      __________________________________________________________________________     EFFECT OF MAT BINDER ON BOARD PROPERTIES                                                                    Stiffness                                                      Urea            (Slope of                                                                           Maximum                                              Resin                                                                               Impregnation    Elastic                                                                             Load                                         Resin   Content                                                                             Level  Caliper                                                                            Density                                                                             Line)                                                                               lb over                                      System  %    lbs./Ft.sup.2 /Side                                                                   In. lb/ft.sup.3                                                                         lb/in.                                                                              10" Span                                     __________________________________________________________________________     Phenol-                                                                        Formaldehyde                                                                            9   .011   .545                                                                               23.5 100  13                                                        .033   .523                                                                               24.7 125  17                                                        .044   .525                                                                               26.6  60  11                                                        .055   .530                                                                               25.4 110  15                                                   15   .011   .536                                                                               24.0 105  17                                                        .022   .534                                                                               24.2 120  18                                                        .033   .515                                                                               25.3 115  16                                                        .044   .520                                                                               24.8 140  16                                                        .055   .529                                                                               25.2 170  23                                           Phenol-                                                                        Formaldehyde/                                                                  Urea-                                                                          Formaldehyde                                                                            9   .011   .494                                                                               25.6 225  44                                                        .022   .492                                                                               25.4 245  43                                                        .033   .496                                                                               25.6 225  30                                                        .044   .487                                                                               25.6 260  41                                                        .055   .492                                                                               26.1 330  46                                                   15   .011   .482                                                                               25.2 190  36                                                        .033   .493                                                                               25.7 240  34                                                        .044   .498                                                                               25.6 290  41                                                        .055   .505                                                                               25.8 310  38                                           Hardboard           .220                                                                               58.6  64  41                                           __________________________________________________________________________

It was further found that boards containing a combination of urea-formaldehyde and phenol-formaldehyde binders are stiffer and stronger than the phenol-formaldehyde bonded boards. The less expensive resin system--that is, the combination of phenol-formaldehyde and urea-formaldehyde produced a thick, lightweight panel that was stiffer and as strong as hardboard having a density of almost 60 pounds/ft³.

The skin thickness measurements for the post-pressed, urea-treated, urea-formaldehyde/phenol-formaldehyde (1:1 weight ratio) bonded mats are set forth in Table VI:

                  TABLE VI                                                         ______________________________________                                         UREA IMPREGNATION                                                                     Skin Thickness at the following urea                                           treatment levels                                                        Resin    .011 lb                                                               Content  urea/Ft.sup.2 /                                                       %        side        .022    .033  .044  .055                                  ______________________________________                                          9       .026        .031    .021  .028  .035                                  15       .025        --      .028  .032  .038                                  ______________________________________                                    

It is preferred to combine the phenol-formaldehyde and urea-formaldehyde resins in a 1:1 weight ratio, and to use a total resin content in the range of 6-20% by weight of total board (dry basis). However, the amount of secondary binder, such as phenol-formaldehyde resin, can vary over a wide range.

OVERLAY PROCESS

It is quite surprising that delamination of the skins from the core can be prevented when the urea is included in overlay fibers in manufacturing the handleable mat, regardless of the particular mat binder. When urea-formaldehyde is the sole synthetic binder in the handleable mat, no delamination occurs so long as the urea is combined in an amount not more than about 20% by weight of the overlay fibers. The urea can be combined in amounts greater than 20% by weight of the overlay fibers so long as the handleable mat contains an additional binder, such as phenol-formaldehyde. Accordingly, when urea is blended in the overlay fibers, as opposed to impregnation into a consolidated handleable mat, phenol-formaldehyde is not necessary as a binder in the core material. It has been found that any suitable binder such as urea-formaldehyde can be used to form the handleable mat without delamination, so long as urea is included by the overlay process during manufacture.

As set forth above, under the impregnation process, the presence of urea in a urea-formaldehyde resin binder board severely retards the ability of the urea-formaldehyde binder to cure at normal pressing conditions. However, since the urea treated face layers are completely converted into a strong, dense skin by the secondary, high-temperature pressing operation (post-press), the urea-formaldehyde resin in the face layers does not have to be completely cured during the manufacture of the handleable mat, regardless of the urea incorporation process. Tests have shown that the final post-pressed panel bending properties are not dependent on the resin content of the face layer. Accordingly, in accordance with a preferred embodiment of the overlay incorporation procedure, the resin content of the face layers does not have to be as high as the resin content of the core layer--it need only be high enough to satisfy the handleability requirement. For example, approximately 4% resin binder in the face layers is sufficient as compared to 8% resin content in the core.

A handleable mat was produced having a basis weight of 1.05 dry pounds per square foot by dry forming the mat in three layers from three separate head boxes with urea blended into the two surface layers, and consolidating to produce a handlable mat having urea in two distinct surface layers but not in the central core. The amount of urea blended into the overlay or surface layers was 12% by weight of overlay fibers. Urea-formaldehyde resin binder was distributed in the core in an amount of 8% by weight and in the face layers in an amount of 4% by weight. The face/core/face weight ratio was 1/3/1. The mat consolidation pressing conditions were as follows:

    ______________________________________                                         Platen Temperature                                                                               280° F.                                               Closing Speed     Fast as Possible                                             Pressure          150 psi                                                      Caliper           0.46"                                                        Time              6 minutes (Total Cycle)                                      ______________________________________                                    

To provide for better surface smoothness and paint holdout, the handleable mat so produced, having a density of 27 pounds/ft.³ was also surface treated with urea in an amount of 11 pounds of urea per thousand square feet by spraying an aqueous solution of urea onto the surface of the mat just prior to post-pressing. The three-layer mat surface treated with urea was then post-pressed under the following conditions without delamination:

    ______________________________________                                         Platen Temperature   550° F.                                            Closing Speed        Fast as Possible                                          Pressure             200 psi                                                   Caliper              0.38" (3/8")                                              Holding Time         30 Seconds                                                ______________________________________                                    

The urea treated, post-pressed fiberboard was compared to Gypsum board, plywood panelling, and hardboard panelling with the results shown in Table VII:

                                      TABLE VII                                    __________________________________________________________________________                                            Work                                    Drawing           Basis           Breaking                                                                            To                                      Identification                                                                               Caliper                                                                            Weight                                                                               Density                                                                             Stiffness                                                                           Load                                         Letter Substrate                                                                             (In.)                                                                              (lb/MFt.sup.2)                                                                       (lb/ft.sup.3)                                                                       (lb/in)                                                                             (lb.)                                                                               (in-lb)                                 __________________________________________________________________________     A      Post-Pressed                                                                   Low Density                                                                    Handleable                                                                     Mat    0.38                                                                               1,070 34   220  75   16                                      B      Gypsum                                                                         Board  0.50                                                                               1,830 44   550  57   10                                      C      Hardboard                                                                             0.22                                                                               1,070 59    64  43   23                                      D      Lauan 3-Ply                                                                    Plywood                                                                        Panelling                                                                             0.14                                                                                 380 32    12  --   --*                                     __________________________________________________________________________      *Testing machine extended to limits w/o failure (maximum load of 16).    

Load/Deflection tests on the panels of Table VII are shown in the drawing. Hardboard of equivalent basis weight (C) breaks at about half the load supported by the low density panel produced in accordance with the present invention (A).

In addition to being stiffer and stroner than hardboard having a much greater density, the surface characteristics of the low density skinned panel are superior with respect to hardness, embossing fidelity, and paint holdout. Accordingly, by post pressing urea treated hardboard of similar higher density (35-65 pounds/ft.³, consolidated fiberboard panels, these properties are similarly improved. "Design fidelity" or simply "fidelity" as used herein is a measure of the accuracy of reproduction of the design of an embossing plate onto the surface of a consolidated mat. The lower density (i.e. 10-35 pounds/ft.³) consolidated mats, urea treated and post-pressed in accordance with the present invention permit deeper embossing patterns because of the lower density. "Paint holdout" is the ability of a panel to keep paint on its surface without a significant amount striking into the panel.

The above prepared handleable mat treated with urea in its surface layers was hot-pressed to panels having calipers or thicknesses of 3/8 inch, 7/16 inch, and 1/2 inch and the final density, stiffness, breaking load, and work to failure of each caliper panel are set forth in Table VIII:

                  TABLE VIII                                                       ______________________________________                                         SKIN THICKNESS ON LOW DENSITY PANELS                                                                                Work  Skin                                               Basis          Breaking                                                                              To    Thick-                              Caliper                                                                              Density  Weight  Stiffness                                                                             Load   Failure                                                                              ness                                (In.) (lb/ft.sup.3)                                                                           (lb/ft.sup.2)                                                                          (lb/in)                                                                               (lb)   (In-lb)                                                                              (In.)                               ______________________________________                                         0.50  26       1.07    280    62      9    .052                                0.44  30       1.07    260    76     14    .051                                0.38  34       1.07    220    79     17    .055                                ______________________________________                                    

The skin thickness was found to be about the same on each panel regardless of panel density.

Attempts to increase the skin thickness by increasing the temperature and time of post-pressing led to the results shown in Table IX:

    ______________________________________                                         D. Post-Pressing Conditions:                                                   ______________________________________                                         1. Platen Temperature                                                                               550° F.                                            2. Closing Speed     Fast as Possible                                          3. Pressure          200 psi                                                   4. Caliper           3/8"                                                      5. Holding Time      30 Seconds                                                ______________________________________                                    

Table XII shows a comparison of properties between the above-formed, low density stress skin panel and hardboard:

                  TABLE XII                                                        ______________________________________                                                         Post-Pressed                                                   Property        Fiberboard Panel                                                                             Hardboard                                        ______________________________________                                         caliper, in.    0.375         0.210-0.220                                      basis weight, lb/ft..sup.2                                                                     1.07-1.12     0.99-1.19                                        gravity         0.54-0.58     0.94-1.00                                        stiffness, lb/in.                                                                              220           65                                               breaking load, lb.                                                                             75            45                                               work to failure, in-lb.                                                                        16            23                                               impact, in.*    10/10         30/14                                            nail head pull through, lbs.**                                                                 80            190                                              nail edge tear out, lbs.**                                                                     110           165                                              24 hour absorption, %***                                                                       60%           24                                               24 hour swell, %***                                                                            14-24         14                                               internal bond, psi****                                                                         20            180                                              ______________________________________                                          *Cumulative drops at 1" increments to height causing failure on top/botto      surfaces.                                                                      **Nail pull and edge tear out tested in accordance with ASTM D1037.            ***Water absorption and swell tested in accordance with ASTM D1037.            ****The internal bond value listed for the low density product is              skin/core interface bond strength.                                       

Buckling comparisons over a 16" span have shown that the low density, 3/8" product is more resistant to buckling than 1/4" hardboard. Table XIII illustrates the buckling resistance of these two products:

                  TABLE IX                                                         ______________________________________                                         EFFECT OF                                                                      TEMPERATURE AND TIME ON SKIN THICKNESS                                                               Skin Thickness, Inch                                     Platen     Holding    Face/Core Weight Ratio                                   Temperature                                                                               Time       2/3        2/2                                           ______________________________________                                         550° F.                                                                            30 Sec.    .069"      .058"                                                    60 Sec.    .062"      .076"                                         600° F.                                                                            30 Sec.    .062"      .070"                                                    60 Sec.    .062"      .074"                                         ______________________________________                                    

The handleable mats of Table IX each had a thickness of 5/8 of an inch, a density of 20 pounds/cubic foot and face/core weight ratios of 2/3, and 2/2. Each mat was bonded with a 1:1 weight mixture of phenol-formaldehyde and urea-formaldehyde resins at a 9% resin content. The face fibers were treated with urea at a level of 55 pounds/thousand square feet/face for the 2/3 face/core weight ratio mat and at a level of 69 pounds/thousand square feet/face for the 2/2 mat. A skin thickness, as recorded in Table IX, represents the average of at least ten measurements. The differences in the skin thicknesses cannot be considered signficant because there is considerable variation and overlapping of the measurements, as shown in the range of skin thicknesses set forth in Table X:

                  TABLE X                                                          ______________________________________                                         RANGE OF SKIN THICKNESS MEASUREMENTS, INCH                                     Platen     Holding    Face/Core Weight Ratio                                   Temperature                                                                               Time       2/3        2/2                                           ______________________________________                                         550° F.                                                                            30 Sec.    .052-.075  .049-.083                                                60 Sec.    .051-.075  .056-.094                                     600° F.                                                                            30 Sec.    .050-.087  .061-.083                                                60 Sec.    .052-.068  .060-.093                                     ______________________________________                                    

It has been found that increasing the resin binder content in the overlay layers, when including urea with the overlay fibers during manufacture of the handleable mat, did not improve the post-pressed panel. It has also been found that post-pressing provides increased stiffness to the product but, to a small extent, weakens the core material during post-pressing. The amount of pressure used in post-pressing should not go beyond 300 psi. Some caliper reduction, however, must result during post-pressing to apply both heat and pressure necessary for thick skin development. Contact (unregistrable) pressure is sufficient for slight caliper reduction. It has been found that as the fibers are compressed during the post-pressing operation, the density increased in the overlay layers and, to some extent, in the core material. Thereafter, the core begins to resist compression. Accordingly, the face or overlay layers will compress more than the core.

A handleable mat having a constant basis weight was cut into equal size boards and compressed to varying calipers to provide samples having different densities in order to determine the optimum thickness and core strength properties of the product. The results are shown in Table XI:

                  TABLE XI                                                         ______________________________________                                         EFFECT OF CALIPER ON BOARD PROPERTIES                                          Final                                Work                                      Product                                                                               Basis            Stiff-                                                                               Breaking                                                                              To    Type                                Caliper                                                                               Weight  Density  ness  Load   Failure                                                                              of                                  (In.)  (lb/ft.sup.2)                                                                          (lb/ft.sup.3)                                                                           (lb/in)                                                                              (lb)   (In-lb)                                                                              Failure                             ______________________________________                                         1/2"   1.08    25.9     284   62      9.1  Tensile                                                                        (Skin)                              7/16"  1.07    29.5     271   76     14.1  Tensile                             3/8"   1.08    34.0     234   79     16.7  Tensile                             ______________________________________                                    

The following example is a preferred embodiment for producing a low density, post-pressed fiberboard panel by the overlay process:

    ______________________________________                                         A. Handleable Mat Forming Conditions:                                          1. Basis Weight       1.05 O.D. lbs/ft.sup.2                                   2. Face/Core Weight Ratio                                                                            2/3                                                      3. Urea Treatment Level in Face Layers                                                               12%                                                      4. Petrolatum Content 1.3%                                                     5. Resin              Urea-formaldehyde                                        6. Resin Content, Face/Core                                                                          4%/8%                                                    B. Handleable Mat Pressing Conditions:                                         1. Platen Temperature 280° F.                                           2. Closing Speed      6 ft/min                                                 3. Pressure           150 psi                                                  4. Final Caliper      0.500"                                                   5. Holding Time       4 min. - conventional                                                          heated platens - and,                                                          1 min. - radio                                                                 frequency heating                                        C. Surface Treatments:                                                         Top Surface of handleable mat is coated with additional                        11 lb/MFt.sup.2 urea, using a 50% solution prior to post-pressing.             ______________________________________                                    

                  TABLE XIII                                                       ______________________________________                                                    Buckle, (in.)                                                                    Post-Press Fiberboard                                             Humidity Conditions                                                                         Panel            Hardboard                                        ______________________________________                                         90° F./70% R.H.                                                                      0.021            0.043                                            90° F./80% R.H.                                                                      0.033            0.202                                            90° F./90% R.H.                                                                      0.042            0.310                                            ______________________________________                                    

POST-PRESS CONDITIONS

A consolidated mat is "post-pressed" at a temperature of at least 525° F. To achieve the full advantage of the present invention, the temperature of "post-pressing" should not exceed 650° F. It is preferred to "post-press" at a temperature in the range of 525°-575° F.

To maintain a relatively low density core material in the fiberboard product in accordance with one embodiment of the present invention, the pressure used in "post-pressing" should not exceed about 300 p.s.i. Pressures can vary depending upon the density of the handleable mat--with lower density mats (i.e. 10-20 pounds per cubic foot) it is preferred to use pressures on the order of 30-150 p.s.i. in post-pressing. With densities of 20 to less than 35 pounds/ft.³ it is preferred to use a "post-press" pressure less than 200 psi. With higher density mats of 35 pounds/ft.³ or greater there is no maximum pressure in post pressing; the pressure is selected for the desired final density of the skinned board. The time of post-pressing is not critical and preferably is in the range of 5-60 seconds. With higher temperatures in the range of 575°-650° F., it is desirable to remove the product from the post-press operation within about 30 seconds to avoid charring or decomposition of the surface skins.

AMOUNT OF UREA

The amount of urea necessary to form a sufficient skin thickness for the purpose of the present invention is at least about 5% by weight of the fibers contacted. That is, when combined by the impregnation process, the weight percent of urea needed is at least 5% based upon the dry weight of the surface material in which the chemical has penetrated. When combined by the overlay process, the weight percent of urea needed is at least 5% based upon the dry weight of overlay material.

The maximum amount of urea which can be incorporated into the handleable mat is dependent upon the presence of urea-formaldehyde resin binders in the mat. Phenolic resin bonded mats can be treated with higher levels of urea than mats bonded with urea-formaldehyde resins. As the urea content of the surface layers increases, the stiffness of the post-pressed panel will increase; also, the skins become more distinct and the post-embossing fidelity and coating holdout improve.

To achieve the full advantage of the present invention, the urea content incorporated into the handleable mat should be in the range of 6-20% based on the dry weight of the material contacted therewith, and preferably in the range of 9-17% by weight. When a binder other than urea-formaldehyde resin, such as phenol-formaldehyde resin, is used in forming the handleable mat, the urea content has no maximum. However, no advantage is realized in terms of skin thickness or other properties by using more than about 20% urea, based on the dry weight of contacted mat material. When urea-formaldehyde is the sole binder used in forming the handleable mat, urea can be incorporated by the overlay process in an amount not exceeding about 30%, based on the dry weight of overlay material.

The product formed by the process of the present invention includes a hard, dense, smooth skin having a density in the range of 40-55 pounds/ft³, generally about 50 pounds/ft³. The composition of the skin cannot be determined by analysis. The base layer or core material underlying the skin can have any desired density, so that lightweight products of varying density can be manufactured having hard skins as thick as 0.070 inch.

Although the present invention is described in terms of particular constituents, and ranges thereof, and manner of making and using the same, it is recognized that departures may be made therefrom within the scope of the invention as defined in the appended claims. 

We claim:
 1. A method of manufacturing a cellulosic fiber containing board having a skin on at least one surface thereof comprising:compressing a fibrous composition, containing cellulosic fibers, under heat and pressure to form a consolidated mat; including urea in at least a surface layer of fibers of said mat in an amount in the range of 5-20% based on the dry weight of said surface layer; and hot-pressing said consolidated mat containing urea in at least its surface fibers, at a temperature of at least 525° F. to form a board having a skin on at least one surface thereof, said skin defined by a thickness of material on said surface of said board having a different composition than the composition of the cellulosic material on which the skin is formed, said skin formed during said hot-pressing of said mat including urea.
 2. A method as defined in claim 1 wherein said consolidated mat is formed by including urea in the surface fibers as an overlay, said method further comprising depositing a first layer of fibers to form a core of fibers; depositing a second layer of fibers, containing urea therewith, over said core of fibers whereby said second layer of fibers intermingles with said core of fibers such that said second layer of fibers containing urea becomes an integral part of said core of fibers; consolidating said first and second layers to form a consolidated mat; and thereafter hot-pressing said consolidated mat at a temperature of at least 525° F.
 3. A method as defined in claim 2 wherein said second layer of fibers contains urea in an amount of at least 5% based on the dry weight of said second layer.
 4. A method as defined in claim 1 wherein said urea containing mat is hot-pressed at a temperature in the range of 525°-650° F.
 5. A method of forming a cellulosic fiber containing board having a relatively dense skin on at least one of its surfaces comprising:impregnating a consolidated cellulosic fiber containing mat, with urea in an amount in the range of 5-20% based on the dry weight of the mat material contacted with urea; and pressing said impregnated mat at a temperature of at least 525° F., said skin formed during said hot-pressing on the surface of said mat impregnated with urea.
 6. A method as defined in claim 5 wherein said impregnated mat is pressed at a temperature in the range of 525°-650° F.
 7. In a method of manufacturing a fiber-board, including the steps of hot-pressing a cellulosic fiber-containing mat to consolidated said mat into a unitary structure having a density in the range of 10-65 pounds per cubic foot, the improvement comprising impregnating at least one surface of said consolidated mat with urea in an amount of at least 5% based on the dry weight of the portion of the mat contacted with said urea and thereafter hot-pressing said impregnated, consolidated mat at a temperature of at least 525° F. for a period of time sufficient to form a hard, dense skin on the surface of said mat, said skin formed during said hot-pressing on the surface of said mat contacted with urea.
 8. In a method as defined in claim 7 including hot-pressing said impregnated, consolidated mat at a pressure not exceeding 300 psi.
 9. In a method as defined in claim 8 including hot-pressing said impregnated, consolidated mat at a pressure in the range of 30-300 psi.
 10. In a method as defined in claim 9 including hot-pressing said impregnated, consolidated mat at a pressure in the range of 100-250 psi.
 11. A method of forming a hard, dense skin on the surface of a fiberboard comprising:contacting the surface fibers of a cellulosic fiber-containing mat with urea; and thereafter hot-pressing said mat after consolidation in a heated press at a temperature of at least 525° F. to form a board having a skin on said surface of said mat, said skin being of different composition than the composition of the material on which the skin is formed, said skin formed during said hot-pressing on the surface of said mat contacted with urea.
 12. A method as defined in claim 11 wherein the mat is hot-pressed at a temperature in the range of 525°-650° F. and at a pressure not exceeding 300 psi.
 13. A method as defined in claim 11 wherein said mat has a density of at least 35 pounds/ft.³ prior to said hot pressing step at a temperature of at least 525° F.
 14. A method of manufacturing a cellulosic fiber-containing board having a skin on at least one surface thereof comprising:forming a cellulosic fiber-containing consolidated mat, having a synthetic binder selected from the group consisting of phenol-formaldehyde, urea-formaldehyde, and starch; impregnating said handleable mat with urea; and hot-pressing said impregnated handleable mat at a temperature of at least 525° F. to form a board having a skin on at least one surface thereof, said skin formed during hot-pressing on the surface of said mat impregnated with urea.
 15. A method as defined in claim 14 wherein said binder comprises starch.
 16. A method as defined in claim 14 wherein said urea-formaldehyde resin comprises no more than 90% by weight of total binders in said board.
 17. A method as defined in claim 14 wherein the consolidated mat contains phenol-formaldehyde resin and urea-formaldehyde resin binders.
 18. A method as defined in claim 14 wherein said consolidated mat is impregnated with urea in an amount in the range of 5-20% based on the total dry weight of mat material contacted by said urea.
 19. A method of manufacturing a cellulosic fiber-containing board having a skin on at least one surface thereof comprising:depositing a first layer of fibers to form a base layer for supporting at least one surface skin thereon; depositing a second layer of fibers over said first layer of fibers, said second layer of fibers including urea in an amount of at least 5% based on the total weight of said second layer of fibers; consolidating said first and second layers in a heated press at a temperature less than 500° F. and at a pressure less than 700 psi to form a consolidated mat and thereafter hot-pressing said consolidated, mat at a temperature in the range of 525°-650° F., and at a pressure not exceeding 300 psi to form at least one skin on said base layer, said skin formed during said hot-pressing on the surface of said mat including urea.
 20. A method as defined in claim 21 further including depositing a third layer of fibers in contact with said base layer, prior to consolidation of said first and second layer to thereby sandwich said base layer between said second and third fibrous layers, said third layer of fibers including urea in an amount of at least five percent based on the total weight of said third layer of fibers; and consolidating said three layers prior to said skin forming hot-press step.
 21. A method as defined in claim 19 wherein said second and third layers contain about half the amount of binder as the base layer.
 22. A method as defined in claim 18 wherein the amount of urea in said second layer is in the range of 9-17% by dry weight of said second layer.
 23. A fiberboard or particle board product comprising a base layer comprising cellulosic fibers and a binder, said base layer having a density in the range of 10-65 pounds/ft.³ ; and an integral skin disposed on at least one face of said base layer, said skin having a density in the range of 40-55 pounds/ft³ and formed by including urea within the surface fibers of said fiberboard, in an amount of 5-20% based on the dry weight of fibers containing urea, consolidating said fiberboard to provide a base layer having a density in the range of 10-35 pounds/ft³, and thereafter heating said contacted surface fibers, under pressure, at a temperature of at least 525° F., to form said skin during hot-pressing on the surface of said mat containing urea.
 24. A method of manufacturing a cellulosic fiber containing board having a skin on at least one surface thereof comprising:depositing a water slurry of cellulosic fibers onto a water pervious support member to form a wet mat; drying said wet mat to less than 8% water, based on the total weight of said mat, to consolidate said mat, including urea in at least the surface layer of fibers of said mat in an amount in the range of 5-20% based on the dry weight of said surface layer; and hot-pressing said consolidated mat containing urea in at least its surface fibers, at a temperature of at least 525° F. to form a board having a skin on at least one surface thereof, said skin defined by a thickness of material on said surface of said board having a substantially different composition than the composition of the material on which the skin is formed, said skin formed during hot-pressing on the surface of said mat including urea.
 25. A method as defined in claim 24 wherein said consolidated mat is formed by including urea in the surface fibers as an overlay, said method further comprising depositing a first layer of fibers to form a core of fibers; depositing a second layer of fibers, containing urea therewith, over said core of fibers whereby said second layer of fibers intermingles with said core of fibers such that said second layer of fibers containing urea becomes an integral part of said core of fibers;consolidating said first and second layers to form a mat having a density in the range of 10-65 pounds/ft.³ ; and thereafter hot-pressing said consolidated mat at a temperature of at least 525° F.
 26. A method as defined in claim 24 wherein said urea containing mat is hot-pressed at a temperature in the range of 525°-650° F.
 27. In a method of manufacturing a fiberboard, including the steps of consolidating a cellulosic fiber-containing mat into a mat, the improvement comprising impregnating at least one surface of said consolidated mat with urea in an amount of at least 5% based on the dry weight of the portion of the mat contacted with said urea, and thereafter hot-pressing said impregnated, consolidated mat at a temperature of at least 525° F. for a period of time sufficient to form a hard, dense skin on the surface of said mat, said skin formed during said hot-pressing on the surface of said mat contacted with urea.
 28. In a method as defined in claim 27 including hot-pressing said impregnated, consolidated mat at a pressure not exceeding 300 psi.
 29. A method of manufacturing a cellulosic fiber-containing board having a skin on at least one surface thereof comprising:forming a consolidated cellulosic fiber-containing mat, and having a cured thermosetting resin binder; including urea in at least a surface layer of fibers of said mat; and hot-pressing said consolidated mat having said cured thermosetting resin, at a temperature of at least 525° F. to form a board having a skin on at least one surface thereof, said skin formed during said hot-pressing on the surface of said mat including urea.
 30. A method as defined in claim 29 wherein said urea is included in said mat prior to consolidation thereof.
 31. A method as defined in claim 29 wherein said urea is included in said mat after consolidation thereof.
 32. In a method of manufacturing a cellulosic fiber-containing board having a skin on at least one surface thereof comprising:depositing a layer of fibers including cellulosic fibers onto a support member to form a loose mat containing cellulosic fibers; consolidating said mat to bond said cellulosic fibers together to form a completed board; contacting at least a surface of said completed board with urea in an amount sufficient to form a surface portion having a density of 40-55 pounds/ft.³ when said contacted surface is hot-pressed at a temperature of at least 525° F.; and hot-pressing said contacted surface at a temperature of at least 525° F. to form a smooth skin on said surface while maintaining a core material, adjacent to said smooth skin of substantially the same density it was prior to said hot-pressing step, said smooth skin portion being formed during said hot-pressing on the surface of said mat contacted with urea.
 33. A method as defined in claim 32 further including drying said urea contacted mat prior to hot-pressing said contacted surface.
 34. A method of manufacturing a cellulosic fiber-containing board having a skin on at least one surface thereof comprising:depositing a layer of fibers including cellulosic fibers onto a support member to form a loose mat containing cellulosic fibers; contacting at least a surface of said mat with urea in an amount sufficient to form a skin on the surface of the mat when said contacted surface is hot-pressed at a temperature of at least 525° F.; consolidating said urea contacted mat to bond said cellulosic fibers together to form a completed board; and hot-pressing said completed board at a temperature of at least 525° F. to form said skin, said skin being formed during said hot-pressing on the surface of said mat contacted with urea.
 35. A method as defined in claim 34 further including contacting a surface of the consolidated board with urea prior to hot-pressing at a temperature of at least 525° F. 